Difference Between Molarity and Molality

Stoichiometry is defined as quantitative relationship between chemical substances in any reaction. Avogadro is known as father of this field of chemistry. The concepts like Molarity and Molality are given by him only. The two terms have similarities but have enough differences that will be talked about in this article to make the concepts clear in the minds of the readers.

Avogadro said that there is a number that represents the number of atoms or molecules of that element in one mole of that substance. Thus one mole of oxygen has a mass of 16 grams as it has n atomic weight of 16. According to Avogadro, one mole of hydrogen would have same number of atoms as one mole of oxygen or any other element. This is known as Avogadro’s number. This information is useful in making solutions of desired concentrations. Molarity and Molality are two units of concentration that are dependent upon the mole concept described above.

Molality is a measure of the concentration of a solution described in terms of number of moles of solute in 1kg of solvent. It has a symbol m. Molality can be easily found by dividing the number of moles of the solute by the mass of the solvent in kilograms. It is pertinent to remember the difference between Molality and Molarity. While both are measures of concentration of a solution, Molality is expressed in terms of mass of the solvent. On the other hand, Molarity is expressed in terms of the volume of the solution. It has the symbol M.

If you have 1kg of solvent with 2 moles of solute dissolved in it, it is a 2m solution. Similarly, 1m solution has 1 mole solute dissolved in 1kg of solvent. So moles/ kilogram is the unit of Molality. One point to be remembered is that while Molality is independent of temperature and does not vary with any change in temperature, Molarity is sensitive to variations of temperature. This is because Molality is a function of mass of the solute which does not change with temperature. On the other hand, Molarity is a function of the volume of the solution.

In brief:

Molarity vs Molality

• Molarity and Molality are measures of concentrations of solutions

• While Molarity is the number of moles of solute present in 1 liter of solution, Molality is the number of moles of solute present in 1kg of solvent.

• The unit of Molarity is moles/liter, whereas unit of Molality is moles/kg

• Molality is independent of temperature while Molarity is dependent upon temperature.